This invention relates to colour reproduction methods wherein colour-separated half-tone negatives or half-tone positives are projected through contact screens on photographic film, for use in preparing printing plates. For convenience, the term "half-tone negatives" is intended to cover in the following specification and claims half-tone positives, as well, wherever applicable.
Priorly, known methods for producing screened film images for use in colour printing are slow and tedious, due to the fact that only one negative at a time is projected through a contact screen onto the photographic film to obtain one specific rate of enlargement. Since the number of half-tone negatives is normally four, i.e. one for each of the colours yellow, blue, red and black, four successive exposures must be made, each one through a contact screen the angular position of which is shifted for each exposure so as to avoid the forming of moire patterns in the end product.
The photographic film is held in close contact with a supporting surface by means of vacuum action exercised through a great number of minute holes connected to a vacuum source. Since all negatives of a set are geometrically alike and are brought to assume an identical position in the negative carrier, their images on the photographic film will become geometrically identical and will thus register perfectly, in spite of any distortion or faulty definition of the projected image caused by the imperfections of the projection lens.
It is obvious that the geometric or definition faults caused by the imperfections of the projection lens will not be self-correcting in this way if two or more associated negatives are projected in the same operation. The resulting images will differ and their registration will necessarily fall short of the standard required for good-quality colour reproduction.